Why does a steel ship float when a steel marble sinks?
After you watchWhy does a steel ship float when a steel marble sinks?
The short answer
A steel ship floats because of its shape, not its weight. Spread into a wide hollow hull, the ship can shove aside a huge amount of water, so it settles down only until the pushed-aside water weighs as much as the ship — then the water holds it up. The same steel packed into a tight marble can shove aside almost no water, far less than the marble weighs, so the up-push is too weak and it sinks.
Try this next
- What if you make the boat walls taller instead of wider? Predict first: will taller walls hold more weight before sinking? Fold a foil boat with high sides, then one with low sides, and load coins into each to see which floods over the edge first.
- What if you float the boat in really salty water? Guess whether the boat rides higher or lower, then stir lots of salt into one bowl of water and compare the same foil boat in plain water next to it.
The whole story
How it works
When something sits in water it has to push water out of the way to make room, and the water pushes back up on it. Floating is a tug-of-war: the object's weight pulls down while the weight of the water it has shoved aside pushes up. A thing floats if its shape CAN shove aside water weighing as much as the thing itself. A wide, hollow ship can shove aside a giant ship-sized chunk of water, so it settles down only until the pushed-aside water weighs exactly as much as the ship — then it stops sinking and floats. A small dense marble can shove aside only a little water, far less than the marble weighs, so the up-push is too weak and it sinks. The metal is identical in both — only the shape changes how much water it can shove aside.
What people get wrong
Most people think heavy things sink and light things float, so it must be about weight. But the very same lump of metal can sink as a tight ball and float as a wide boat, without adding or removing one bit. What really decides it is whether the shape CAN shove aside water weighing as much as the object: the boat can, so it settles until the up-push matches its weight; the ball cannot, so it sinks.
The catch
Spreading metal into a wide hollow boat lets it float, but only while it stays open and watertight — let water flood in over the side or through a hole and the hollow fills with heavy water, so it shoves aside no extra space and sinks like the ball. A tight solid ball is small and tough, but it can never float because it shoves aside almost no water for its weight.
Questions kids ask
If the ship and the marble are both made of steel, why does only the ship float?
Because the ship's wide hollow shape CAN shove aside far more water. Floating depends on whether a shape can push aside water that weighs as much as the object, not just on the weight of the metal. The ship can, so it settles down only until the pushed-aside water weighs as much as the ship and the water holds it up; the marble can shove aside only a tiny bit, far less than it weighs, so it sinks.
Does a heavier ship sink lower in the water?
Yes. Adding cargo makes the ship heavier, so it sinks a little deeper until it has shoved aside extra water that weighs as much as the new cargo. As long as the pushed-aside water can still match its total weight, it keeps floating — just lower.
Why do ships sink if they get a hole in them?
A ship floats because its hollow shape holds air and shoves aside lots of water. If water leaks in, it fills the hollow with heavy water instead of light air, so the shape no longer shoves aside extra space. Now the water it pushes aside weighs less than the flooded ship, and it sinks.
What does it mean to push water aside?
Anything you put in water has to make room for itself by moving water out of the way. A wide shape makes room for a lot of water; a small tight shape makes room for very little. The more water a shape shoves aside, the harder the water pushes back up.
Talk about it
- Before we try it — guess which one floats, the foil ball or the foil boat, and tell me why you think so.
- Both are the exact same piece of metal. What do you think the boat shape is doing that the ball can't?
- If you wanted to make this boat carry the most coins without sinking, how would you fold it?
For grown-ups
An object floats when it displaces a weight of water equal to its own weight (Archimedes' principle); the upward buoyant force equals the weight of the water displaced. A solid steel lump sinks because steel's density (~7800 kg/m³) far exceeds water's (~1000 kg/m³) — it cannot displace its own weight before it is fully submerged. A hull encloses a large volume of air, so the average density of steel plus air drops below water's, letting the ship displace enough water to balance its weight. Same metal, same mass; the shape changes the displaced volume, not the weight.
Keep going
What else makes you wonder?
- If the same metal can float or sink just by changing shape, what other things might float if you reshaped them?
- How deep would the boat sink if you piled rocks into it, and what would make it finally go under?
- Submarines can choose to float or sink whenever they want — how do you think they switch between the two?